BMI Calculator for Kids
Enter your child's age, sex, height, and weight to see their BMI-for-age percentile using CDC growth-chart data. The result includes the weight-status category (underweight, healthy, overweight, or obesity), the percent of the 95th percentile, and a plain-language interpretation. Works for children and teens ages 2 through 19. Switch between metric and imperial units at any time.
Why kids’ BMI is different from adult BMI
For adults, BMI is compared against fixed cut-offs (18.5, 25, 30). For children and teens, those same cut-offs cannot be used because body fatness changes with age and differs between boys and girls. A BMI of 18 means something completely different for a 4-year-old versus a 14-year-old. Instead, the CDC uses sex-specific growth charts built from a large national reference sample to express a child's BMI as a percentile: how their number compares to same-age, same-sex peers measured in the 1970s and 1980s. This approach accounts for normal developmental variation during growth.
How the percentile is calculated
This calculator uses the LMS (Lambda-Mu-Sigma) statistical method developed by Cole and Green (1992) and adopted by the CDC for their 2000 growth charts. Three smoothed parameters - L (a Box-Cox power transformation), M (the median BMI), and S (the coefficient of variation) - are tabulated for each sex at each month of age from 24 to 240 months. The formula transforms any measured BMI into a z-score relative to that month's reference distribution, and the z-score is converted to a percentile using the standard normal distribution. This captures the natural skewness of BMI distributions in children more accurately than a simple mean-and-SD approach.
CDC weight categories for children and teens
The CDC defines four weight-status categories. Underweight is below the 5th percentile: fewer than 1 in 20 same-age peers weigh less, which may indicate inadequate nutrition or an underlying health condition. Healthy weight (5th to below 85th percentile) covers the broad middle of the distribution. Overweight (85th to below 95th percentile) means the child weighs more than 85 to 95 percent of peers. Obesity is at or above the 95th percentile. Severe obesity is further divided into Class 2 (at or above 120% of the 95th percentile or BMI 35 or above) and Class 3 (at or above 140% of the 95th or BMI 40 or above), thresholds introduced by the American Academy of Pediatrics in 2023.
Limitations and when to see a doctor
BMI-for-age is a screening tool, not a diagnosis. A child at the 88th percentile is not automatically unhealthy, and a child at the 15th percentile is not necessarily thin. Muscle mass, bone density, puberty timing, and ethnicity all affect how a BMI number should be interpreted. Always share these results with the child's pediatrician, who can review them alongside full growth history, physical findings, dietary habits, and activity levels. Children whose percentile is outside the healthy range should be evaluated clinically before any intervention is considered.
CDC BMI-for-age weight-status categories (ages 2-19)
| Percentile range | Weight category | Recommended action |
|---|---|---|
| Below 5th | Underweight | Consult healthcare provider |
| 5th to <85th | Healthy weight | Maintain healthy habits |
| 85th to <95th | Overweight | Discuss with healthcare provider |
| 95th or higher | Obesity | Evaluation recommended |
| >=120% of 95th | Severe obesity | Prompt evaluation recommended |
Based on sex-specific CDC 2000 growth charts. Percentile compares a child against same-age, same-sex peers.
Frequently asked questions
What is a healthy BMI percentile for a child?
The CDC considers the 5th to below 85th percentile to be the healthy-weight range for children and teens aged 2 through 19. This wide range reflects normal variation in body composition during growth. A child consistently inside this band who is growing along their own curve is generally considered to have a healthy weight for their height and age.
Is BMI the same for boys and girls?
The formula for computing raw BMI (weight divided by height squared) is the same for everyone, but the percentile interpretation uses separate growth charts for boys and girls. Boys and girls have different average BMIs at each age, especially during puberty, so using sex-specific reference data is important for a valid comparison.
What does the percent of the 95th percentile mean?
The percent of the 95th percentile is calculated as the child's BMI divided by the 95th-percentile BMI for their age and sex, then multiplied by 100. A value of 100% means exactly at the obesity threshold, 120% is the cutpoint for severe (Class 2) obesity, and 140% is the Class 3 threshold. It gives clinicians a finer measure of degree within the obese range.
Can BMI predict a child's future health?
Research shows that children with BMI in the overweight or obese range have a higher likelihood of remaining in those categories as adults, and face elevated risks for conditions such as type 2 diabetes, high blood pressure, and sleep problems even in childhood. However, BMI is one data point, not a destiny: healthy habits, growth trajectory, and clinical context matter much more than any single snapshot.
Why does this calculator use ages 2 to 19?
The CDC growth-chart reference data covers children and teens from 24 months (2 years) to 240 months (20 years). BMI is not typically used below age 2, where weight-for-length is the preferred indicator. Above age 19, adult BMI cut-offs (18.5, 25, 30) apply directly and no longer need age- or sex-specific percentile adjustment.
Is this calculator appropriate for all ethnicities?
The CDC 2000 growth charts were built from a nationally representative U.S. sample and are the recommended standard for U.S. clinical practice. Some research suggests that cardiometabolic risk may start at a lower BMI percentile in certain Asian populations, similar to the lower adult cut-offs some authorities use. If ethnic-specific thresholds are relevant, discuss them with the child's healthcare provider.